The worst tsunami in history happened 20 years ago. Here's how it unfolded

The Indian Ocean tsunami that occurred on Christmas Day in 2004 was one of the most severe natural disasters of its kind in human history. On December 26, 2004, at 7:58 AM local time, an earthquake with a magnitude of at least 9.1 struck off the northern coast of Sumatra, Indonesia, in the Indian Ocean.

According to the Australian Institute for Disaster Resilience, tsunami waves began hitting the coasts of northern Sumatra and the Nicobar Islands within 15 minutes of the earthquake. Approximately two hours later, the tsunami reached Sri Lanka, India, and Thailand. About seven hours later, the waves even struck Mauritius and the east coast of Africa.

The U.S. National Oceanic and Atmospheric Administration estimated that wave heights reached 51 meters in the Aceh province of northern Sumatra, Indonesia. These waves were about 30 times the height of an average Australian, equivalent to seven times the height of a two-story house. The disaster caused at least 227,000 deaths, with the affected area spanning over a dozen countries from Indonesia to the coast of Africa, with Indonesia, Sri Lanka, Thailand, and India being the most severely impacted. Among the victims were 26 Australians.

The Australian Institute for Disaster Resilience called it the worst tsunami ever and one of the ten most severe earthquakes on record. Dr. John Dawson, Community Safety Lead at Geoscience Australia, stated, "This is absolutely the largest tsunami that has resulted in the most deaths on record, going back to 365 AD." He also emphasized that what made this tsunami unique was that many people died far from the epicenter, "with over 52,000 people dying more than 1000 kilometers from the epicenter, which is unusual in a disaster."

Dawson stated that the magnitude of this earthquake was very rare, with the only modern earthquake of a similar scale being the one that occurred in the Tohoku region of Japan in 2011, which led to the Fukushima nuclear disaster. "In 2004, there was no tsunami warning capability in the Indian Ocean, and the region was completely unaware, which is one of the reasons why so many people died—the waves arrived without warning," he said. "We cannot predict earthquakes, but they are part of a process that repeats every few hundred years, so we expect there will be more, which is why we maintain our tsunami monitoring capability."

Since the Indian Ocean earthquake, Geoscience Australia and the Bureau of Meteorology have been collaborating to monitor earthquakes and potential tsunamis. This year marks the 20th anniversary, and they hope that this event has not faded from people's memories. "The risk with very rare but catastrophic events is that it fades from people's memories, and they forget," Dawson said. "That's why it's also important to reflect on the 2004 event and say that this could happen again. Are our communities prepared for it?"